Warning regarding throughput limitation imposed due to RWIN settings and increased latency.In TCP networking the RWIN (TCP Windows size) is the amount of data that can be accepted by acomputer without being acknowledged. If the sender of the data hasn’t received an acknowledgementfor the first packet it has sent, then it will stop and wait and once this wait exceeds a limit, it mayretransmit. This is how TCP achieves reliable data transfer. Even with no data loss in a network, thisRWIN limitation can impose a throughput limit according to the following formula depending on theoverall Round Trip Time (RTT):Throughput <= RWIN (bytes) / RTT (seconds)For example, on a DSL system running in fast mode, with a round trip time of 8ms and a defaultRWIN size of 65535bytes, the throughput is limited to a maximum of:Throughput max =65535/0.008 = 8.19MbpsIf this line is placed in interleaved mode with an interleaving delay of 16ms, then potentially the roundtrip time could be increased to between 32-40ms dependant on the delay imposed by the DSLAM. Ifwe recalculate with the round trip time at 40ms, thenThroughput max = 65535/0.04 = 1.64MbpsIn order to address this restriction, it is possible on most systems to increase the default RWIN size toimprove this. For example, increasing the RWIN to 327600 would return the maximum throughputback to the original figure:Throughput max = 327600/0.04 = 8.19MbpsIt should also be noted that this restriction is imposed only to a single TCP stream. It is possible toutilise the entire available DSL bandwidth through the use of multiple TCP streams and or UDP traffic.
